Thermochromism comprises the property of a material to change its colour reversibly or irreversibly as a function of the temperature. This can be effected both by changing the intensity and/or the wavelength maximum. Examples and theoretical backgrounds are described comprehensively in Chromic Phenomena by Peter Bamfield (The Royal Society of Chemistry, 2001) or in Thermochromic Polymers by A. Seeboth and D. Lötzsch (Encyclopaedia of Polymer Science and Technology, John Wiley & Sons, Inc. 2003).
The application for polymer flat films comprising polyethylene, polypropylene, polyester derivatives or a multiplicity of further polymers and combinations thereof in multilayers is nowadays omnipresent in virtually all areas of life.
It is common to polymer materials with general thermochromic properties that they generally have a thermally induced colour or transparency change. Thus the colour change with simultaneous intensity reduction is described in WO 02/08821. EP 1 084 860 claims the switching of the colour effect, based on a donator-accepter colourant system, with at least two further components. The change in colour is effected within broad temperature ranges, such as between −50° C. to 120° C. or −40° C. to 80° C. The entire colourant system can be inserted in the polymer matrix optionally also in the form of microcapsules with a diameter of approx. 50 μm.
The developments disclosed in U.S. Pat. No. 5,527,385 comprise in addition organic hydrazide, sulphur or phosphorous compounds which are intended to improve for example the lack of light stability. The thermal stability is hereby negatively influenced.
The production of polymer thermochromic materials with the help of printing (laminating) of a thermochromic colour is a practical solution for some requirements and wishes of the packaging industry but does not achieve the set objective. Thus according to US 20020037421, glasses are printed with a colour for use as sun protection or, corresponding to U.S. Pat. No. 4,121,010, polymers are coated with a thermochromic colour, comprising sulphates, sulphides, arsenic, bismuth, zinc and other metals and oxides thereof. As a result, both the field of use is greatly restricted and also as a result of the required additional coating (printing technique) cost-reducing continuous technology cannot be used. In addition, it should be mentioned that the complexity of printing or labelling is often out of all proportion to the production costs.
The cited solutions in the mentioned patent specifications are not suitable for producing large-area polymer films with thermochromic properties. This applies also to the invention described in EP 1 157 802. Here, during extrusion-blow moulding, thermochromic pigments are added only in partial regions of the wall thickness of a plastic material container. This takes place by adding a reversible thermochromic pigment in the form of strip-shaped inserts. The thermochromic pigments require in addition additional thermostable pigments and are added to or placed on the extrudate before leaving the extrusion head. Thorough mixing of thermochromic material and polymer carrier material is correspondingly not possible and is not sought here either.
Thermochromic pigments which are faded with a defined temperature effect and are commercially available are used here preferably. As is known, the pigments are added in the form of master batches in extruder technology. It has however been known for many years that master batches based on microencapsuled liquids, e.g. used in thermochromic inks, or on liquid crystal components, e.g. used in paints or films as heat sensors, do not have sufficient mechanical stability for use in extruder technology where the polymer melt is subjected to exceptional mechanical requirements at simultaneous increased temperature when passing through the extruder screw.